Last year I posted "Summer Daze." In that post I detailed a new pattern of atmospheric behaviour centred on high pressure over Greenland, this pattern is new since 2007, and as I showed is a robust change since the 1950s in NCEP/NCAR data.
Well, it's happened again this year...
The pattern I showed consisted of strongly anomalous high pressure over Greenland surrounded by a ring of tendency to low pressure anomalies. I used NCEP/NCAR monthly analysis to examine it, source data. As with all atmospheric patterns, in some years it was stronger than others, and there was interannual variability in the details of the pattern, but taking a succession of years it was seen to be a common pattern. The robustness of this pattern and its unusual nature is highlighted by taking the average pressure fields for batches of years and subtracting one batch from another. The pattern of the post 2007 period survives this method, still being present for any 5 year period subtracted from the period 2007 to 2011. As could be seen from the original Summer Daze post, this process shows that the pattern doesn't occur regularly in the past in the way it has done since 2007.
This year the pattern is textbook perfect.
What spurred the original post was trying to figure out whether there was an impact on UK summer weather. The plot above shows purple over the UK, that's a negative deviation of pressure which is indicative of the low pressure dominated summer we've had this year.
And what have been the practical consequences of this low pressure dominate summer? The wettest summer in 100 years. Met Office. At this juncture it might seem reasonable to say I'm putting too much weight on just one year. But as I show in the original post, the above pattern holds, more or less, for every summer since 2007.
Then there's the Central England Temperature Series. This is the oldest instrumental temperature series in the world, more here, and I'm using it because I don't have access to series of UK average temperatures. However it serves its purpose.
I've used the seasonal average of maximum temperature, here, which is used because it's daytime maximum temperature (minimum is set in the night) that affects our perception of whether a summer is hot or cold. And it's my own anecdotal memory of summers since 2007 that I'm testing here, actually it's not just me - the idea of recent years cool wet summers seems to be gaining more traction in the UK. It's just that most people don't connect it with the Arctic.
So here is the CET series from 1975.
There's a gradual warming trend from 1975 to 2006 which drops off after 2007, this continues this year. Using the trend from 1975 to 2006 it's possible to work out the differences from the trend, known as residuals.
It's clear that there is a significant drop from the trend after 2007.
2012 continues all of this behaviour reinforcing an atmospheric pattern that has held since 2007. What's significant about 2007?
In 2007 the Arctic lost a quarter of it's area.
Source: Cryosphere Today.
2007 also saw a substantial loss of volume.
Source data: PIOMAS.
In 2007 the Arctic suffered a massive loss of sea ice area and volume.
After that there was a largescale shift in Arctic atmospheric configuration.
At the same time there was also a succession of cool, damp, low pressure dominated summers for the UK.
Is this a coincidence? Is it F...